In one aspect, this invention relates to an apparatus for drying solid materials which contain water. In another aspect, the invention relates to a method of drying solid materials.
The invention is particularly applicable to the drying of lignite, although it should be understood that the invention may be equally suitable for the drying of other coals, rubber products, ceramic materials, wood, or any other moisture laden material from which water removal is desirable. Lignite is a low ranking subbituminous coal which has a relatively low heating value of typically about 7,000 B.T.U. per pound, about half of higher ranking coals. This low heating value is primarily attributable to lignite's high moisture content, which ranges from 15% to 60% by weight. Thus, it is desirable to remove water from lignite to increase its heating value accordingly. Another reason for drying lignite is that very moist lignite cannot properly be gasified in a coal gasifier. As is well known, coal is frequently fed into a coal gasifier, such as a Lurgi reactor, wherein the coal is contacted with steam and oxygen to produce a useable fuel gas.
Past methods of drying lignite, typically involving subjecting the lignite to hot gases, cause considerable decrepitation of lignite lumps. Such decrepitation is thought to be caused primarily by rapid vaporization of water from the surface of the lignite, which is accompanied by closing of pore openings, shrinkage, etc. Decrepitation of lignite lumps is particularly undesirable since resulting very fine lignite particles can plug a coal gasifier so as to reduce the gasifier efficiency or render it inoperable. In addition, fine lignite particles or dust produced by prior drying techniques present a considerable fire and explosion hazard.